TOUCH PSYCHOPHYSIOLOGY L Ngyessy PPKE 2010 Haptic exploration

TOUCH PSYCHOPHYSIOLOGY L. Négyessy PPKE, 2010

Haptic exploration of local shape Static stimuli 1 -2 mm 2, 8 mm min. 0, 5 mm : 3% 0, 17 mm 2

Thresholds I Braille dots’ height: 500 µm 3

Vibrating stimuli OPTACON 6 x 18 = 108 blunt pins 230 Hz 4

Thresholds II d’ Discriminabitlity (d’) of complex waveforms for low frequencies 5

The 4 channel model - psychophysics Threshold 6

The 4 channel model - neurophysiology Sensory Afferents Conduct. : 35 -70 m/s (Hand scan: 60 -80 mm/s) 7

Receptive field organization (finger pad) SAI RA P SAII ? 1 mm 2 0. 82 mm 2 diffuse Aff. denz. 100 cm-2 150 cm-2 350/ finger ? (low) Diverg. (RF area) 4 -16 (5 mm 2) 1: 1 Converg. 1: 1 2 -7 1: 1 Adequ. stim. Strain energy density (point, edge, curve) Slip, load force High freq. vibration Skin stretch function Form, texture Grip control, fine discr. Distant events, tool use Hand shape RF size 8

Form perception: dots, edges, curves Braille reading Aperiodic grating SAI channel 9

Receptive field characteristics relevant to form perception RAI & SAI channels 10

Surround supression skin mechanics 11

Perception of texture: roughness 12

Fine texture perception: SAI spatial variation* *Mean absolute difference in firing rates between SAI afferents with RFs separated by ~2 mm 13

SAI spatial variation code for fine textures (0. 2 -1 mm) 14

Summary of form & texture perception coding of spatial features Evidences of SA 1 specialization for the representation of spatial information: n SA 1 responses to stimulus elements on a surface are independent of the force of application. n SA 1 -receptive fields grow minimally (relative to RA receptive fields) with increasing indentation depth. n SA 1 afferents possess a response property, surround suppression, which confers response properties similar to those produced by surround inhibition in the central nervous system. This response property is a consequence of sensitivity to strain energy density, not a synaptic mechanism. n SA 1 spatial resolution is affected minimally by changes in scanning velocity at velocities up to at least 80 mm s– 1. n SA 1 afferents are at least ten times more sensitive to dynamic than to static stimuli. n SA 1 responses to repeated skin indentation are practically invariant: the variability is about one impulse per trial regardless of the number of action potentials evoked. n The RA system has greater sensitivity but poorer spatial resolution and 15 limited dynamic range.

Vibrotactile perception: flutter, vibration 16

RA channel Periodic Stimulus Firing rate Periodic St Aperiodic St Periodicity (IS interval) Firing rate monkey ideal obs. Periodic+aperiodic St Only firing rate Thresholds ratios: psychometric/neurometric thresholds firing rate periodicity 17

P channel ideal obs. of St motion 10 nm skin motion at 200 Hz 18

Response to vibrating stimulus 19

Summary of vibrotactile perception - coding of temporal features P channel n intense filtering (at nearly 60 d. B per decade) of low-frequency stimuli n respond to stimuli less than 100– 150 Hz with a phase-locked, Poisson discharge, therefore a whole population firing randomly but at a rate proportional to the instantaneous stimulus amplitude can represent the stimulus waveform accurately RA channel n RA neurons of S 1, like their afferent fibers, fire periodically, in phase with mechanical oscillations n RA neurons modulate their firing rates as a function of the stimulus frequency n Flutter is encoded by firing rate of RA neurons 20

Adaptation Peripheral mechanisms 21

Time course of adaptation and recovery 22

RA interference in spatial processing 23

Tool use 24

Coding object size 25

Grasping and manipulation 26

Grasping and manipulation 27

SUMMARY • The 4 channel model of vibrotactile discrimination • RF correlates of the 4 channel model • Elements of form perception • Texture (roughness) perception • Vibrotactile perception • Object manipulation 28